J 



152 THE PRINCIPLES OF SCIENCE. 



more slowly in glass than in air^. More recently, in 1850, 

 Fizeau and Foucault independently measured the velocity 

 of light in air and water by a revolving mirror, and found 

 that the velocity is greater in air r . There are indeed a 

 number of other points at which experience decides 

 against Newton, and in favour of Huyghens and Young. 

 Euler rejected the Corpuscular theory because particles 

 of matter moving with the immense velocity of light 

 must possess great momentum, of which there is no 

 evidence in fact 5 . Bennet concentrated the light and heat 

 of the sun upon a body so delicately suspended that an 

 exceedingly small amount of momentum must have been 

 rendered apparent, but there was no such effect*. This 

 experiment, indeed, is of a negative kind, and is not 

 absolutely conclusive, unless we could estimate the mo- 

 mentum which Newton's theory would require to be 

 present (see vol. ii. p. 45) ; but there are other difficulties. 

 Laplace pointed out that the attraction supposed to exist 

 between matter and the corpuscular particles of light, 

 would cause the velocity of light to vary with the size of 

 the emitting body, so that if a star were 250 times as 

 great in diameter as our sun, its attraction would prevent 

 the emanation of light altogether u . But so far as experi- 

 ence shows, the velocity of light is uniform, and inde- 

 pendent of the magnitude of the emitting body, as it should 

 be according to the undulatory theory. Lastly, Newton's 

 explanation of diffraction or inflection fringes of colours 

 was only plausible, and not true ; for Fresnel ascertained 

 that the dimensions of the fringes are not what they 

 would be according to Newton's theory. 



q Airy's 'Mathematical Tracts,' 3rd edit. pp. 286-288. 



r Jamin, ' Cours de Physique/ vol. iii. p. 372. 



s Euler's 'Letters/ vol. ii. Letter XIX. p. 69. 



* Balfour Stewarjb, 'Elementary Treatise on Heat/ p. 161. 



u Young's 'Lectures on Natural Philosophy' (1845), vol. i- P- 361. 



